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Explorer 3



Explorer 3 (1958 Gamma) was launched in conjunction with the IGY by the U.S. Army (Ordinance) into an eccentric orbit. The objective of this spacecraft was a continuation of experiments started with Explorer 1. The payload consisted of a cosmic ray counter (a Geiger-Mueller tube), and a micrometeorite detector (erosion gauge). The Explorer 3 spacecraft was spin stabilized and had an on-board tape recorder to provide a complete radiation history for each orbit. It was discovered soon after launch that the satellite was in a tumbling motion with a period of about 7 s. Explorer 3 decayed from orbit on June 28, 1958, after just over 93 days of operation.

Spacecraft and Subsystems

Explorer 3 was essentially the same as Explorer 1, a 2.03 m long, 0.15 m diameter cylinder and nosecone that comprised the fourth stage of the Jupiter-C launch vehicle. With a mass of 14.06 kg, it was about 0.1 kg heavier than Explorer 1. The base of the cylinder held the Sergeant solid-fuel rocker motor. The sub-carrier oscillators and Mallory mercury batteries for the low power transmitter were in the upper part of the nose cone. Below these was the low power (10 mW, 108.00 MHz) transmitter for the carrier and sub-carrier signals and an air gap for the nose cone, which acted as the low power system antenna.

Below the nose cone was the detector deck, holding the Geiger-Mueller counter tube for the cosmic ray experiment, the command receiver, for recorder interrogations, high power playback transmitter (60 mW, 108.03 MHz) for interrogation response, cosmic ray experiment electronics, Mallory mercury batteries for the high power transmitter, and 0.23 kg, 5.7 cm diameter magnetic tape recorder (which was not on Explorer 1). A gap for the high powered antenna and a heat radiation shield were between the payload and the rocket motor. The high power system used the instrument and motor cases as a dipole radiating antenna. The micrometeorite detectors were arranged in a ring around the cylinder near the bottom of the spacecraft. Unlike Explorer 1, Explorer 3 had no radial whip antennas and no acoustic micrometeorite detector.

The launch vehicle was a Juno 1, a variant of the three-stage Jupiter-C with an added fourth propulsive stage, which in this case was the Explorer 3. The first stage was an upgraded Redstone liquid-fueled rocket. The second stage comprised a cluster of eleven Sergeant solid-fuel rocket motors and the third stage held three Sergeants.

Mission Profile

Explorer 3 was launched from the Cape Canaveral Missile Test Center in Florida on 26 March 1958 at 17:38 UT (12:38 pm EST). The spacecraft was injected into a 186 x 2799 km altitude orbit with an inclination of 33.38 degrees and a period of 115.7 minutes at 17:45:06 UT. The first successful interrogation of the command readout system was achieved 3 minutes later by the Antigua Minitrack station.

The spin rate (about the long-axis) of Explorer 3 on its initial orbits was 10 revolutions per second (rps), but over the next 10 days this slowed to 2 rps. The angle of precession also increased over this time, so that after 10 days the spacecraft was effectively tumbling with a period of 7 seconds. The experiments and communications systems operated nominally until early May.

On May 7 the micrometeorite detector data showed two grids had broken, indicating micrometeorite impacts. On May 8, telemetry channel 5 (temperature) on the low power transmitter ceased operating, and on May 9 the modulator on the low power transmitter was not operating and at 10:22 UT interrogation on the high power transmitter became intermittent. On May 10 the carrier signal disappeared. On May 11 the high power transmitter went silent, and on May 12 the low power transmitter also failed. On May 14 the carrier signal reappeared, without the subcarrier tones, but disappeared for good on May 16. The interrogation response was seen briefly on 21 May, the last interrogation response came on 24 May 1958 at 06:52 UT. The last signal from the high power system came on June 5. Since the two transmission systems were completely independent, it was speculated that the micrometeorite detections may have been due to the Eta Aquarids meteor shower, and that other impacts may have occurred at the same time and damaged spacecraft systems, leading to failures of the two transmission systems. The orbit of Explorer 3 decayed on 28 June 1958 and it reentered the atmosphere.

Alternate Names

  • 00006
  • Explorer3
  • 1958 Gamma

Facts in Brief

Launch Date: 1958-03-26
Launch Vehicle: Juno
Launch Site: Cape Canaveral, United States
Mass: 14.1 kg

Funding Agency

  • Department of Defense-Department of the Army (United States)


  • Space Physics
  • Astronomy

Additional Information

Questions and comments about this spacecraft can be directed to: Dr. David R. Williams



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